EEMB 102 Week 10

The replacement of dinosaurs by mammals at the KT event (CJ event) mostly means that they outlasted them – what helped mammals pass through the KT extinction?

Mammals remained small, less than 5kg, until after the KT extinction. When the meteor impact occurred, it caused the atmosphere to heat to intense temperatures for several days. This likely killed many of the terrestrial plants and animals, but small mammals that could retreat to burrows were more likely to survive.

Which reptile groups survived the PT extinction, and what did their descendants become?

Therapsids (which became mammals), Archosaurs (crocs, dinosaurs, birds), and Lepidosaurs (lizards and snakes).

Discuss the supposition that dinosaurs suppressed mammalian radiations for 140 my – how?

Mammals evolved in the early Jurassic but stayed mostly small and few in number for 140 my, then radiated drastically into different ecological niches and morphologies following the KT extinction. This explosive success after the extinction of dinosaurs, combined with the classic idea that mammals are inherently “superior” to dinosaurs, were the reasons for the assumption that mammals were suppressed by dinosaur competition. This was for reasons like predation and ectothermy vs endothermy.

There were many lineages of Mesozoic mammals – what were their shared features?

Mesozoic mammals were small and nocturnal, being insectivores or fungivores. They used hearing and olfaction (expanding the nasal cavities and lining them with turbinate bones covered in sensory neurons) to find their prey rather than vision. They also evolved complex surfaces on their teeth, meaning they could chew their food more thoroughly

There are 10 major biological differences between dinosaurs and mammals – explain respiration, hearing, cognition, viviparity, dentition, and nitrogenous waste metabolism. Which favor archosaurs and which favor mammals?

1. Respiration: mammals have a tidal, blind pouches system. Archosaurs have extensive air sacs and a series of parabronchial channels. Archosaurs are favored because while they breathe in and out, they have continuous air flow and therefore almost 2x efficiency with gas exchange. 

2. Hearing: dinosaurs have a one bone ear system, while mammals have a three bone linkage system. Mammals also have a larger surface area ratio from tympanum to the oval window in the cochlea. Mammals are favored, because their system allows for amplification of small sounds and some prevention of damage due to loud sounds.

3. Cognition: archosaurs have mostly corpus striatum (little cross linkage of neurons). Mammals have cerebellum with layers of interconnected neurons. In general, this translates to mammals being better at problem solving and adaptability.

4. Viviparity: archosaurs all lay eggs (oviparity) while mammals give live birth (viviparity). Embryonic respiration sets a size limit for eggs, while mammals can give birth to young 10-30% of their maternal mass. Mammal babies only survive if their mother survives, with the trade off of getting nutrients provided by the mother throughout development. Live born young are larger and more likely to survive, but oviparity allows for relatively low cost young. 

5. Dentition: dinosaurs had homodont dentition, aka rows of similar teeth in simple forms. Mammals have heterodont dentition, with tons of diversity of size/shape/function of teeth this is considered a mammalian key innovation, so they are favored.

6. Nitrogenous waste metabolism: synapsid amniotes (mammals) combine 2 ammonia to make urea. Diapsid amniotes (reptiles, birds, dinosaurs) take it one step further by combining urea to make uric acid. This favors dinosaurs because uric acid uses no metabolic water.

Dinosaurs were mostly small in the Triassic, but reached giant size in mid late Mesozoic, but in that period there were very few mid sized dinosaur species – what might be the explanation for a lack of rabbit-to-pony sized dinosaurs?

The mid size dinosaur niche was filled by the young of larger dinosaurs, which presumably took long periods of time to mature. The juveniles were thus replacing mid-sized dinosaurs.

What advantages do placental mammals have over (a) monotremes and marsupials, and (b) all archosaurs?

Placental mammals have large, well developed young which are often able to be coordinated and active right after birth, something monotremes and marsupial young are not. Archosaurs’ young were often very tiny compared to their adult forms due to egg contraints, and their young get no additional nutrients provided during their development in the egg.

What types of evidence are cited in support of an impending 6th mass extinction? Are we there yet, or not?

There are many potential drivers for a human mediated mass extinction. Two views exist; the first view that at some point, in the future, extinction rates of species could reach up to 70-96%, and humans would go extinct by the time we hit 30-40% biodiversity loss, and the second view that we could somehow manage to live without a lot of species by some virtue of technology. Some of the causes that would lead to extinctions can be regulated (carbon emissions), but other practices are harder (engrained cultural practices).

List the Court Jester and Red Queen drivers of a modern era mass extinction.

1. Court jester drivers

   1. Climate change – warming, altered precipitation, and rising CO2

   2. Toxic pollution – heavy metals, pesticides, plastics, radioactive waste, and other contaminants

2. Red queen drivers

   1. Habitat loss and degradation – agriculture, logging, urbanization and coastal development

   2. Invasive species and diseases – competition, predation, and pathogen spread

   3. Direct human caused mortality – overhunting, overfishing, and other forms of exploitation

   4. Human population growth and societal pressures – resource depletion, zoonotic disease emergence, and potential socioeconomic instability 

Why are rising CO2 levels cited as a prime concern in extinctions?

CO2 level increases in the atmosphere lead to elevated global temperatures due to greenhouse effect. This causes extinction of plants and animals with narrow thermal tolerance (that are unable to move habitats) and rising sea level.

Habitat loss (land conversion) and exotic species affect biodiversity (how and where?)

Habitat loss occurs by conversion of wildlands to agricultural areas. Deforestation was originally an issue in temperate regions, but now is expanding to the tropics as well. The effects include modifications to climate well as chemicals from runoff (pesticides, etc.). Exotic feral mammals have become a big problem in areas like Australia which prey on and outcompete native wildlife. This has led to especially high extinction rates in Australia

Review the evidence for human predation vs. climate change as the dominant factor in megafaunal extinctions.

1. The fossil record shows a correlation between human arrival to Australia and North America, and South America and the extinction of many larger mammals. The same can be said about birds after human arrival to various islands like Hawaii and New Zealand. After human arrival to Madagascar and within 500 years nearly all animals larger than 10kg were gone. Fossils of these species also include butchering and burning marks. 

2. A counter in the North American argument is the impact of the Younger Dryas event. This was a period of global cooling in an otherwise warming period when glaciation was ending that coincided with extinction of some megafauns and Native American clovis culuters. A comet impact is believed to be the cause by some, but others argue for glacier meltwater affecting polar climates. It is unclear why only large mammals would suffer in an event like this.

Are negative effects at the base or at the top of a food chain more important? Why?

Even though effects at the top of the food chain are most discussed, this is a red herring. The effects at the base of the food chain are much more important, because these effects upscale to affect all the subsequently higher levels of the food chain. For example, the decline of insects, which form the foundations of many terrestrial food webs and are important in energy transfer, “marches” up the food chain.

Why are so many extinctions (birds and snails especially) on oceanic islands?

Remote oceanic islands often have several species of native birds, and many lose the ability to fly due to negative selection by an environment with things like cyclones. These species are vulnerable to predation.

Genetically reconstructing extinct species may be possible, but where do they fit in?

These extinct species – even if they are brought back – may not fit into a world where other species have filled in the niches they once used to occupy.

Explain how chytrid fungi are causing amphibian declines worldwide.

Chytrid fungi are skin parasites that feed on keratin and compromise respiration and water retention in infected animals. It likely emerged from northeast Asia (where species are not particularly susceptible to infection) but has spread globally to many more vulnerable species. They are spread by global commercial trade.

Global epidemics are a thing of the past thanks to modern medicine – agree or disagree

Humans might be the biggest exploitable adaptive zone, and an immense array of pathogens (microbes, viruses, fungi, etc.) are constantly mutating and will inevitably bypass the human immune response. A lot of these are zoonotic diseases, which are transmitted via livestock and the consumption of exotic animals and wildlife. Escapes from labs may also happen. With advances in modern travel, these diseases can spread rapidly.

What are zoonotic diseases and where are the global hotspots?

Zoonotic diseases are diseases which are transmitted directly from wildlife to humans. Current global hostpots for zoonotic disease transmission are Africa and East Asia, specifically when wild bats and primates are used for food.

The end game in a modern mass extinction is most likely to be societal collapse – list and explain the factors that would enter in.

The effects of climate change on biodiversity, ecosystem services, human mortality, and state fragility (inability to manage risks) may ultimately add up to cause mass extinction and societal collapse. An immediate factor is overpopulation, combined with dwindling resource supplies.

Would a 6th mass extinction be an accident, a crime, or an inevitability? Explain.

1. Humans have set into motion many things like climate change, habitat loss, and proliferation of parasites and invasive species, some which are now exponential in their impacts. Additionally, our infrastructure includes maintenance of things like dams and nuclear silos which require constant upkeep. Humans have killed off most megafauna that would be our predators, and now dominate the Earth. We are now a relatively untapped adaptive zone for disease evolution due to our population size and interconnectivity.

2. The 6th mass extinction will not be an accident, it is arguably a crime, and is likely an inevitability thanks to a “death by thousand cuts” of current extinction drivers combined with the fallout of disease driven human societal collapse.